1. Technical Field
Embodiments described herein relate to a wiring substrate and a method of manufacturing the wiring substrate.
2. Description of the Related Art
A method of electrically connecting pads of a wiring substrate and electrodes of a semiconductor chip to each other through solder bumps is widely known. In this method, a problem has occurred in which cracking occurs on the interface between the solder bump and the pad due to electromigration according to solder bump miniaturization and an increase in the current flowing through the solder bump in recent years and this cracking causes defective bonding.
As a method of reducing the electromigration, it is effective to increase the opening area of the pad. However, it is difficult to increase the opening areas of all pads due to the miniaturization of the design. For this reason, measures to increase the opening area of only a pad connected to a circuit, through which particularly large current flows, have been taken. That is, a plurality of kinds of pads with different opening areas are formed on the wiring substrate (see e.g., JP-A-2008-227355).
However, if electrode terminals are formed by mounting solder balls with the same diameter simultaneously on the pads with different opening areas or by forming solder bumps using a screen printing method, the height of the electrode terminal at the pad having a small opening area is larger than the height of the electrode terminal at the pad having a large opening area. As a result, since the electrode terminal whose height is smaller does not reach a semiconductor chip, there is a problem in that the wiring substrate and the semiconductor chip cannot be connected to each other.
In order to solve the above-described problem, a method of forming electrode terminals with the same height by mounting solder balls with different diameters on the pad having a large opening area and the pad having a small opening area may be considered. In this method, however, it is necessary to mount solder balls multiple times using a mask. For this reason, if the opening area becomes very small, high-accuracy mask alignment is required and therefore this is not practical.
As another method to solve the above-described problem, a method of forming electrode terminals with the same height by making the position of the pad having a small opening area lower than the position of the pad having a large opening area (by making the position of the pad having a small opening area deeper than the position of the pad having a large opening area) may be considered. In this method, it is possible to form electrode terminals with approximately the same height by mounting solder balls with the same diameter on the pad having a large opening area and the pad having a small opening area.
However, it is necessary to open only the pad having a small opening area by resist patterning, perform etching using an etchant, and make the pad having a small opening area deeper than the pad having a large opening area. For this reason, if the opening area becomes very small, high-accuracy alignment is required at the time of resist patterning and therefore this is not practical.